Three-phase static inverter



s sheets-sheet 1 Filed May 20, 1960 BRAHM Aug. 11, 1964 c. B. BRAHMTHREE-PHASE sTATlc INVERTER Filed May 2o, 1960 5 Sheets-Sheet 2 INVENTOR CHARLES B. BRAHM BY 'gna-m21 AGENT Aug. 11, 1964 c. B. BRAHMTHREE-PHASE sTATrc INVERTER 3 Sheets-Sheet 3 Filed May 20, 1960 4INVENTOR BRAHM CIHARl-ES B- AGENT M No. M M m.

United States Patent Office 3,144,599 Patented Aug. 11, 1964 3,144,599THREE-PHASE STATlC INVERTER Charles B. Brahm, Ellington, Conn., assignerto United Aircraft Corporation, East Hartford, Conn., a corporation ofDelaware Filed May 20, 1%0, Ser.. No. 30,657 12 Claims. (Cl. 321-7) Thisinvention relates to an improved power supply system, and particularlyto a static inverter which will convert direct current into three-phasealternating current.

An object of this invention is to provide a novel threephase staticinverter and an accurate phase control in which the three phases areprecisely displaced 120 from each other.

A further object of this invention is to provide a threephase staticinverter which is accurate over wide variations in temperature andvoltage, but which is simple, efficient and inexpensive.

Another object of this invention is to provide a threephase staticinverter which allows accurate control of each phase independently, andin which losses are at a minimum.

These and other objects and a fuller understanding of this invention maybe had by referring to the following specification and claims, read inconjunction with the drawings, in which:

FIG. 1 is a block diagram of the static inverter; and

FIG. 2 shows the waveforms which operate the 400 c.p.s. multivibrator ofFIG. l; and

FIG. 3 shows the circuitry of the phase splitter of FIG. l; and

FIG. 4 is a vector diagram of the response of the phase splittingcircuit of FIG. 3; and

FIG. 5 shows. the circuitry of the power stage and regulator of FIG. l.

Referring to the block diagram of the static inverter of FIG. 1, atransistorized tuning-fork oscillator operating at 1200 c.p.s. isutilized as the primary reference source which governs the frequencyaccuracy and phase symmetry of the output of the inverter. Oscillatorsare available which will maintain the frequency constant to within .005%of 1200 c.p.s. over a temperature range from 55 C. to +125 C.

The choice of a tuning fork instead of a commonly used crystaloscillator as the reference source is advantageous because of thesimplification in circuitry which it provides. In order to obtain highaccuracy in the static inverter, it would be necessary to operate acrystal oscillator at a high frequency and to count down by means of 5or 6 binary stages to 400 c.p.s. The reference oscillator necessarilybecomes quite complex in that for each binary stage, at least twotransistors and several other components are required. A furthercomplication may be encountered if the crystal oscillator requires anoven for temperature stabilization. This would not only add complexity,but would also consume power and appreciably lower the overallefficiency of the inverter. The tuningfork oscillator 10 provides asimple, straightforward means for obtaining constant frequency, sincethe entire oscillator employs fewer transistors and requires less powerthan a corresponding crystal oscillator.

The reference frequency of 1200 c.p.s. was chosen mainly because it iswell above the maximum anticipated vibrational frequency of aircraft,for which the inverter is particularly adapted. The reference sourceshould, therefore, be immune from harmful infiuence by vibration. The1200 c.p.s. frequency proves ideal not only for frequencysynchronization, but for maintaining phase symmetry of the output.

The waveform of the 1200 c.p.s. reference oscillator 10 is converted bypulse former 12 into a series of pulses capable of synchronizing amultivibrator 14, the natural frequency of which is 400 c.p.s. Acountdown of three is thus accomplished. As will be described later, the1200 c.p.s. pulses are also used for initiating the generation of threeoutputs spaced 120 electrical degrees apart. The prime advantage of thissystem is simplicity and relative immunity from temperature effects,since phase and frequency are independent of the shifts in properties ofcomponents.

Multivibrator 14 may be a basic transistorized freerunning or a stablemultivibrator, which is a simple twostage resistance-capacitance coupledtransistor amplifier with the output of the second stage coupled backthrough a capacitor to the base of the first stage transistor. Thesemultivibrators are well known in the art, and will not be described herein detail.

The method by which synchronization of multivibrator 14 occurs isillustrated in FIG. 2. The output of the pulse former 12 is a series ofpulses A through M, and the multivibrator output waveform is showndirectly above these pulses. To follow the synchronization process,consider that pulse A occurs and drives one transistor into conduction.The transistor continues to conduct for a time determined by circuitparameters. During this time pulse B occurs, but has no effect since thetransistor is already conducting. Next, the transistor becomesnon-conducting for a period again determined by circuit parameters.During this off time, pulse C occurs but is ineffectual since the baseof the transistor is biased far below cutoff, and the pulse is ofinsufficient amplitude to cause conduction. Pulse D, however, is able toinitiate conduction and occurs three reference oscillator cycles afterpulse A. The frequency of multivibrator 14 is thus synchronized at 400c.p.s. by positive pulses originating by the tuning fork oscillator 10.The frequency of multivibrator 14 is thereby rendered insensitive totemperature changes. Changes in the capacitance or resistance ofmultivibrator 14 which determine the free-running frequency will resultin only slight changes in waveform, since the point at which conductionis initiated in each transistor of the multivibrator is determined bythe synchronizing pulses. The output of the multivibrator 14approximates a square wave.

A simple low-pass filter 16 is used between the output of multivibrator14 and a phase splitter 18 to eliminate harmonics from the square waveoutput of multivibrator 14. The filter 16 may consist only of a seriesinductor followed by a shunt capacitor. Once the harmonics are removed,it becomes possible to shift phase and to produce, from the single phaseinput, three output signals phased 120 electrical degrees apart.

In order to provide the three phase output from the static inverter, itis necessary to phase split the frequency controlled output ofmultivibrator 14. Phase splitter 18 converts the incoming signal into athree-phase output, symmetrical with respect to both amplitude andphase.

A schematic diagram of phase splitter 18 is shown in FIG. 3. Phase A' isderived from the secondary of transformer 20 and is 180 out of phasewith the input signal A. A phase lag of 60 from the incoming signal isprovided to phase B by series resistor 22 and shunt phase triangularwaveform. ln order to arrive at the desired waveform, the output ofphase splitter 18 is used to trigger three monostable muitivbrators 30,32, and 34. The square wave outputs of the multivibrators are, in turn,integrated to provide the proper triangular wave shape. lt should benoted also that the square wave outputs of multivibrators 30, 32, and34, which are 120 out of phase with each other, may be used to provide athree-phase output voltage directly, without the use of regualtingcircuitry. For example, a simple filter may convert the square waveoutputs of the multivbrators directly into a sinusoidal output, and theresulting three phase output will have accurate frequency and phasecontrol. It is obvious, however, that the addition of circuitry forregulating the voltage and power output of each phase independently willgreatly enhance the utility of such a power supply.

The multivibrators 30, 32, and 34 are of the conventional one-shot typewhich do not operate until fired by an incoming signal of sufiicientamplitude. As may be seen in FIG. 1, each of the three output phasesfrom phase splitter 18 is connected to one of the monostablemultivrators through lines 36, 3S and 40.

Pulses from the puise former 12, which are 120() c.p.s., are directed toeach of the three monostable multivibrators through lines 42, 44, and46. These 1200 c.p.s. pulses are superimposed on the three-phase outputsfrom phase splitter 18. The signal used to trigger each of themonostable multivibrators is a composite waveform made up of a 400c.p.s. sine wave plus a 1200 cycle pulse.

The multivibrators are each fired at a 400 c.p.s. rate, phased 120apart. The input to each multivibrator from phase splitter 18 issufiicient to trigger the multivibrators and the 1200 c.p.s. pulses areused are used to fire each multivibrator in synchronism with the 1200c.p.s. pulse and thus prevent deviation from phase symmetry. Assumingthat a given 1200 c.p.s. pulse causes multivibrator 30 to fire at acertain instant, the next 1200 c.p.s. pulse causes multivibrator 32 tofire 1/3 of a 400 c.p.s. pulse later or 120 after multivibrator 30.Similarly, a third 1200 c.p.s. pulse will fire multivibrator 34, 240after multivibrator 30 and 120 after multivibrator 32. Although thetechnique of frequency synchronization is employed, the ratio ofreference frequency to desired power frequency, that is, 1200 c.p.s. to400 c.p.s., gives immunity from temperature-induced drifts in phaseangle by providing phase synchronization.

The output of each monostable multivibrator is converted into atriangular waveform. This may be done by simple integrating circuits 48,50, and 52 consisting of a series resistor and a shut capacitor.

The regulating portion of the static invertor will be described firstwith reference to the block diagram of FIG. 1. The triangular outputfrom the integrators is connected to a driver amplifier stage 54, 56,and 58, which raises the signal to a level sufficient for driving theoutput power stage. The driver amplifiers consist of two stages, adriver preamplifier operated as a saturated amplifier to provide arectangular wave output which drives a push-pull saturated driveramplifier. The output from the driver stages is a rectangular wave whichis coupled to a push-pull power stage 60, 62, and 64. The power stage isa class B amplifier operated in the switching mode. The output of thepower stage has a rectangular waveform which is width modulated toprovide regulation. Each power stage operates into single phase outputtransformers 66, 68 and 70, and the transformers are connected to filternetworks 72, 74, and 76. The filters may be shunt-m-derived networks andtransform the rectangular power stage outputs into a smooth sine waveoutput. The filter also acts as a transient suppressor in the A.C. lineprotecting the power stage from load switching transients. The outputsfrom phases A, B, and C are 120 out of phase with each other. Thefrequency of the output has been kept constant by the synchronizationprocess in multivibrators 30, 32, and 34. Since the amplitude of theoutput waves is dependent upon the load on each phase, regulation isprovided by sensing the output waveform and feeding a D.C. error signalback to the input of driver stages 54, 56, and 5S and varying the DC.level of the triangular input to the driver stages. FIG. 1 showsregulators '78, 30, and 82 as performing this operation.

FIG. 5 shows in detail the operation of the regulators. The operationwill be described only for phase A but it will be understood that thesame operations are applicable to each phase. Integrator 48 supplies thetriangular wave to transformer 100, and the secondary of the transformeris coupled through diodes 114, and 116, 117 to the base junctions of PNPtransistors 102 and 104. This is the driver pre-amplifier stage. Thesetransistors are operated as saturated amplifiers in order to provide arectangular output. Voltage and power regulation are provided by varyingthe length of time each transistor conductors. This is known as pulsewidth modulation.

The triangular waveform is the signal which switches the transistors 102and 104 into and out of conduction. The length of conduction time can bevaried if the base line of the triangular wave is moved up or down withrespect to a fixed firing line. A D.C. component can be applied to thetriangular wave and varied upward to increase conduction time or loweredto shorten conduction time. An increase in conduction time is equivalentto an increase in power. A source of positive D C. voltage 106 providesa positive potential to the emitters of the transistors through atemperature compensating diode 108. A path is provided through resistor110 and capacitor 112 and through the secondary winding of transformer100, diodes 114, and 116 and resistors 118 and 120 to ground, thuskeeping the base connections of transistors 102 and 104 more negativethan the emitters, and consequently the transistors will be turnedslightly on at all times but with limited current fiow. This allows thetriangular input to turn the transistors fully on more easily. When thetriangular input is applied to the bases of the transistor, one of thetransistors will turn on fully and saturate, while the other transistorwill be turned off. When the triangular input wave reverses, thetransistor which was conducting will now be turned off, while the offtransistor turns fully on. The length of time that each transistor isconducting is determined by the triangular input as modified by the D.C.bias provided to the bases of each transistor. lf the bias is low, thatis, if the base-emitter junction of the transistor is only slightlyforward biased, the triangular input wave will turn the transistors ononly for a short time, whereas when the bias is high, the transistorwill be turned on for a much longer period of time by the sametriangular input wave. The output of the transistor is rectangular andis connected to the driver amplifier stage 54. As will be describedlater, a D.C. error voltage is applied through lines 122 and 124 acrossresistor 110 and filtering capacitor 112 to vary the D.C. bias oftransistors 102 and 104, and thus regulate the conduction time of eachtransistor. The pulse width output of the transistors 102 and 104 willalways be slightly less than lf, however, the D.C. error voltage callsfor conduction time greater than 180, diode 126 will conduct and preventthe conduction time of the transistors from being more than 180. Diode126 will thus protect the system from overloading.

Although transistors 102 and 104 have been described as PNP transistors,it is obvious that the principles described apply also to lNPNtransistors as well.

The driver amplifier 55 may be another stage of pushpull saturatedtransistors turned on and off by the rectangular output from transistors104 and 102. The output from the driver amplifier is connected to thepower stage 60, which may be a transistorized class B amplifiertransformer 134. The output voltage is stepped down to obtain a valuecompatible with the reference voltage to be described later. Secondarywinding 136 is connected through a resistor 138 to a full wave rectifier140 where the A.C. output voltage is converted to direct current. Therectifier output is filtered at 142 and the D.C. signal is connectedthrough line 144 to one side of primary winding 146. This D.C. signalwill be proportional to the amplitude of the sinusoidal output voltageand is compared across the primary winding 146 to a reference voltageand any deviation of the output voltage from this reference signal willbe measured and fed back to change the bias of transistors 102 and 104to increase or decrease their conduction time and thus modify the outputvoltage to eliminate the deviation. v

The reference voltage is provided by using the original output acrossterminals 128 and 130 and sensing this output across transformer 134 bysecondary winding 147. The terminals of secondary winding 147, terminals148 and 150, will supply this regulated output through resistors 152 and154 in order to divide the voltage. A pair of double-anode Zener diodes156 and 158 are placed back to back across the secondary winding andprovide additional coarse regulation and clipping of the sine wave. Anadditional pair of zener diodes 160 and 162 are placed across the lineto clip the wave again and to provide additional fine regulation. Zenerdiodes 160 and 162 in the fine regulating stage are operated at aconstant current at which their temperature coefficient of voltage ispractically zero. The regulation results in a square wave output acrossprimary winding 164. The square wave is coupled through transformer 166to a full wave rectifier circuit 168. Filtering may be necessary at thispoint. The D.C. output of this rectifier will supply the regulatedvoltage to all three phases, so that it is not necessary to duplicatethe circuitry for the other two stages in order to provide a source ofreference voltage. The source of sinusoidal voltage is used as theoriginal reference voltage because it is regulated itself, thussimplifying the other stages of regulation provided by the Zener diodes.

The D.C. reference voltage is conducted to one side of a diode modulatorbridge circuit 170 and a source of alternating voltage which may be fromwinding 147 of transformer 134 is used to change the bridge from a highimpedance to a low impedance at a 40() cycle rate. The source of A.C.voltage is indicated at terminals 148 and 150. Diode bridge circuit 170is thus used as a switch, and the D.C. reference voltage is conductedthrough the bridge circuit when the bridge circuit is in its conductingor low impedance state. This reference voltage is conducted through line172 to the other side of primary winding 146 to which the D.C. outputsignal is connected. If the D.C. output signal is the same as thereference voltage no signal will result across primary winding 146;however, if the output signal is above or below its desired value, theD.C. signal will differ fromy the D.C. reference voltage and arectangular A.C. output will result across primary winding 146, thedirection and magnitude being proportional to the error between thedesired output as indicated by the reference voltage and the actualoutput. This rectangular wave is coupled through transformer 174 to A.C.amplifier 176 and to transformer 178.

More than one stage of A.C. amplification may be necessary to bring theerror voltage to a level at which it can regulate effectively. A pair ofphase sensitive diode demodulators 186 and 182 are supplied by areference A.C. voltage from secondary winding 184. The error voltageoutput from A.C. amplifier 176 through transformer 178 will be sensed bysecondary windings 186 and 188 and applied to the demodulators 180 and182. The operation of diode demodulators is well known in the art andwill not be described in detail here. The demodulators will convert theA.C. error voltage to a D.C. signal whose magnitude and direction areproportional to the deviation of the sinusoidal output voltage from thedesired value, and this D.C. voltage will be conducted through lines12.2 and 124 to modify the bias of transistors 102 and 104 and thuschange the voltage level of the triangular wave and the conduction timeof the transistors which will result in regulating the output voltageacross terminals 128 and 130. The sinusoidal output is thus keptconstant regardless of the load or changes in operating conditions.

Voltage and power regulation of the static inverter is assured by thepulse width modulation method of regulation independent of thefrequency, and frequency regulation is assured by the novel method ofsynchronization. The inverter as described will thus provide a simpleand efiicient method of producing a three-phase power supply system.

The pulse width modulator and regulating system is more fully describedand claimed in a copending application entitled Pulse-Width Modulator,application Serial No. 30,545, by the same inventor and filed on evendate with this application.

While the invention has been described in its preferred embodiment, theinvention is not limited thereto and changes in the details ofconstruction and the combination and arrangements of parts may beresorted to without departing from the spirit and scope of theinvention.

I claim:

1. A power supply system comprising means to produce a first signal at aselected frequency, a multivibrator synchronized by said first signalfor producing a second signal having a frequency which is a sub-multipleof said first signal, converting means connected to receive said secondsignal and to produce therefrom a plurality of phase-displaced voltagesignals, a plurality of output circuits, means for applying at least oneof said plurality of phase displaced voltage signals to each of saidoutput circuits to actuate said output circuits, and means forsimultaneously applying said first signal to said output circuits tosynchronize the actuation of said output circuits.

2. A three-phase power supply system comprising means to produce a firstsignal at a selected frequency, means synchronized by said first signalfor producing a second signal having a frequency which is a sub-multipleof said first signal, circuit means receiving said second signal andproducing therefrom first, second and third voltage signals, each ofsaid voltage signals being displaced in phase from the other saidvoltage signals, first, second and third output circuits, means forapplying at least one of said voltage signals to each of said outputcircuit to actuate said output circuits, and means for simultaneouslyapplying said first signal to said output circuits to synchronize theactuation of said output circuits.

3. A three-phase power supply system as in claim 2 in which the outputcircuits are monostable multivibrators triggered by said voltagesignals.

4. A three-phase power supply system as in claim 3 in which said circuitmeans for producing voltage signals from said second signal comprises aphase splitting circuit including a transformer having a primary windingand a secondary winding, and a phase lead circuit and a phase lagcircuit each connected across said transformer primary winding.

5. A three-phase power supply system as in claim 4 in which the means toproduce said first signal includes an oscillator.

6. A three-phase power supply system as in claim 5 in which said meansfor producing said second signal is a free-running multivibratorsynchronized by the output from said oscillator, the second signal beinga rectangular wave which is fed to the primary transformer winding ofsaid phase splitting circuit.

7. A power supply system comprising means to produce a series of timedpulses, means receiving said series of pulses and producing therefrom aplurality of voltage signals phase displaced 120 from each other, aplurality of monostable multivibrators, means applying one of saidvoltage signals to each of said multivibrators to trigger saidmultivibrators and to produce an output signal from said multivibrators,and means simultaneously applying said series of timed pulses to each ofsaid multivibrators to synchronize the triggering of saidmultivibrators.

8. A power supply system as in claim 7 and including means forregulating the amplitude of the output signal from each saidmultivibrator.

9. A power supply system as in claim 8 in which the output signal fromeach multivibrator is converted into a triangular wave.

10. A power supply system as in claim 9 and including a pulse widthmodulator for receiving each said triangular wave and producing arectangular signal having a pulse width proportional to the voltagelevel of said triangular wave.

11. A power supply system as in claim 10 and including means to convertsaid rectangular signal into a sinusoidal voltage.

12. A power supply system as in claim 11 and including means for varyingthe voltage level of said triangular Wave as a function of the amplitudeof the sinusoidal voltage.

References Cited in the le of this patent UNITED STATES PATENTS2,596,167 Philpott May 13, 1952 2,623,203 Demuth Dec. 23, 1952 2,648,722Bradley Aug. 11, 1953 2,898,412 Linn Aug. 4, 1959 2,899,572 Skelton etal Aug. 11, 1959 2,916,687 Cronin Dec. 8, 1959 OTHER REFERENCESTransistorized Three-Phase Power Supplies, by W. Brannian, published inElectronic Industries (January 1959); pages 02-05 relied on.

1. A POWER SUPPLY SYSTEM COMPRISING MEANS TO PRODUCE A FIRST SIGNAL AT ASELECTED FREQUENCY, A MULTIVIBRATOR SYNCHRONIZED BY SAID FIRST SIGNALFOR PRODUCING A SECOND SIGNAL HAVING A FREQUENCY WHICH IS A SUB-MULTIPLEOF SAID FIRST SIGNAL, CONVERTING MEANS CONNECTED TO RECEIVE SAID SECONDSIGNAL AND TO PRODUCE THEREFROM A PLURALITY OF PHASE-DISPLACED VOLTAGESIGNALS, A PLURALITY OF OUTPUT CIRCUITS, MEANS FOR APPLYING AT LEAST ONEOF SAID PLURALITY OF PHASE DISPLACED VOLTAGE SIGNALS TO EACH OF SAIDOUTPUT CIRCUITS TO ACTUATE SAID OUTPUT CIRCUITS, AND MEANS FORSIMULTANEOUSLY APPLYING SAID FIRST SIGNAL TO SAID OUTPUT CIRCUITS TOSYNCHRONIZE THE ACTUATION OF SAID OUTPUT CIRCUITS.